At the present time thin-film solar cells (TFSCs) and panels represent one of the largest segments of the photovoltaic industry, mainly due to their low cost, possibility of using large and flexible substrates, and improved thermal properties. The most popular materials for TFSCs include hydrogenated amorphous silicon (αSi:H), microcrystalline or nanocrystalline Si, CdTe/CdS, and CuInGaSe (CIGS). A general description of these TFSCs is given in Physics of Semiconductor Devices, Second Edition, by S. M. Sze, John Wiley and Sons, 1981, pp. 825 to 830.
An αSi:H-based TFSC is typically made in the form of a single αSi:H-layer that contains a PIN structure or in the form of stacked αSi:H-layers wherein each layer consists of a PIN structure and wherein a connection between layers is made through a tunnel junction (TJ). The latter design is commonly known as a tandem solar cell (TSC) or a multijunction solar cell (MJSC). Underneath the αSi:H top layer, a TSC may also include layers of microcrystalline or nanocrystalline Si. A tandem design is generally preferred because it exhibits higher output voltage (Vout) and higher power conversion efficiency (PCE) compared to a single-layer cell. When a TSC is made exclusively of αSi:H layers, the tandem design is used to reduce impact from the Staebler-Wronsky (SW) effect. A detailed description of an αSi:H-based TSC is given in “Amorphous silicon-based photovoltaics—from earth to the “final frontier” by Jeffrey Yang, et al., in Solar Energy Materials & Solar Cells”, v. 78, pp. 597 to 612. It should be noted that all known TSCs are designed as “vertical” structures, wherein the top PV cell is formed on the surface of the bottom PV cell, i.e., on the light-receiving side of a TSC and absorbs solar radiation of high photon energy (i.e., with shorter wavelengths), while the bottom PV cell (or cells) is formed beneath the top PV cell and absorbs radiation of the low photon energy (longer wavelength).
The general fabrication process for a conventional stacked thin-film tandem solar cell (hereinafter referred to as a “TFSC”) is described, e.g., in U.S. Patent Application Publication No. 20070240759 published in 2007 (inventor, P. Borden). According to one aspect, the module includes separate terminals for the respective layers in the stack, thus allowing the current in each layer to vary without sacrificing efficiencies gained because of varying bandgaps. According to another aspect, a processing method according to the invention includes forming interconnects for each layer using etch and deposition processing, including forming separate interconnects for each respective layer, the interconnects of which can be coupled to respective sets of terminals.
U.S. Patent Application Publication No. 20100065099 published in 2010 (inventor, S. Neumann, et al) discloses a method of manufacturing a monolithically interconnecting electrical devices that isolates and interconnects the contacts of neighboring electrical devices, such as thin film PV cells, without damaging surrounding materials. The method comprises the following steps: providing a first conducting layer covering at least one semiconductor layer covering a second conducting layer covering a substrate; forming a first trench penetrating through at least the first conducting layer; forming a second trench such that the second trench penetrates through at least one semiconductor layer and the second conducting layer; forming a third trench such that the third trench penetrates through at least one semiconductor layer; at least partially filling the second trench with a resistive material; at least partially filling the third trench with an electrically conductive material such that it provides an electrical current pathway from the first electrical device first conducting layer to the second electrical device second conducting layer, wherein at least the second or third trench is within the boundaries of the first trench.
U.S. Patent Application Publication No. 20100065099 published in 2010 (inventor: S. Neumann, et al.) discloses a method of manufacturing a monolithically interconnecting electrical devices that isolates and interconnects the contacts of neighboring electrical devices such as thin film PV cells, without damaging the surrounding materials. The method comprises the following steps: providing a first conducting layer covering at least one semiconductor layer covering a second conducting layer covering a substrate; forming a first trench penetrating through at least the first conducting layer; forming a second trench such that the second trench penetrates through at least one semiconductor layer and the second conducting layer; forming a third trench such that the third trench penetrates through at least one semiconductor layer; at least partially filling the second trench with a resistive material; at least partially filling the third trench with an electrically conductive material such that it provides an electrical current pathway from the first electrical device first conducting layer to the second electrical device second conducting layer, wherein at least the second or third trench is within the boundaries of the first trench.
U.S. Patent Application Publication No. 200900301543 published in 2009 (inventor, D. Reddy, et al) discloses a process for making a thin-film photovoltaic device by providing a substrate with a plurality of holes, depositing a metal electrode layer on each side of the substrate for creating bottom and back electrodes, scribing a portion of the metal layer from the circumference of the holes to electrically isolate the holes from the bottom electrode, and scribing the bottom and back electrodes longitudinally for forming adjacent cells. As a result, the adjacent cells acquire electrical contact with one another through at least one contact between the bottom electrode of one cell and the back electrode of an adjacent cell through at least one hole that is positioned between the bottom scribe and the back electrode scribe. The process is accomplished by depositing an absorber layer and a transparent conductor layer. As a result, a thin-film photovoltaic device with monolithic integration and backside metal contacts is obtained.